electronics & telecommunications1 entc455 wireless transmission systems entc 455
TRANSCRIPT
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Basics of Wireless Networks
Harvey Lehpamer – Transmission Systems Design Handbook for Wireless NetworksChapter 2 – pages 5 to 45
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Basics of Wireless Networks
Historical Background – The Cellular Concept Wireless Local Loop (WLL) Cellular Systems
Analog (FDMA) – AMPS Digital:
TDMA: NA-TDMA (IS-136), GSM CDMA (IS-95)
1G, 2G, 2.5G, 3G,….. Satellite Networks Microwave Systems Bluetooth
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Historical Background 1946 – the first public mobile telephone service was introduced in
25 major American cities Transmitter tower that serves a metropolitan area (> 50 km) Mobile users Several channels available
Half-duplex mode 120 khz of bandwidth per channel (voice)
1960’s Channels
30 Khz of bandwidth per channel (voice) IMTS – Improved Mobile Telephone Service
Full duplex, auto-dial, auto-trunking phone systems Spectrally innefficient
1976 – New York – 12 channels that could serve only 543 paying customers
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Cellular Concept (proposed in 1968, implemented late 70’s) Metropolitan areas were divided into cells Small geographic area: cells Low power transmitters Frequency re-use Each cells operating on a set of frequencies that differed
from the frequencies of adjacent cells.
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Cellular Concept
Figure 3: Mobile Telephone System Using a Cellular Architecture
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Cellular Concept – Frequency reuse
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7 Cell clusterCluster Size N = 7
Frequency reuse factor = 1/7Each cell contains one-seventh of thetotal number of available channels
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Frequency reuse concept
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A cell cluster isreplicated over thecoverage area
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Cellular Network
Figure 8: Cellular System Components
Mobile Switching Center
Up Link
Down Link
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Method of locating co-channel cells in a cellular system
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Number of cells per cluster NN can only have values thatSatisfy the equation:
N = i2 + i j + j2
To find the nearest co-channelneighbors of a particular cell: 1) move i cells along any chainof hexagons 2) turn 60 degrees counter-clockwise and move j cells
N = 7i = 2j = 1
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Example – To understand how frequency re-use improves capacity If a total of 5 MHz of bandwidth is allocated to a
particular FDD wireless system which uses 25 KHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per metropolitan area.
25 KHz
5,000 KHz
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Solution Total bandwidth = 5 MHz Channel bandwidth = 25 KHz x 2 simplex channels = 50
KHz/duplex channel Total available channels = 5,000/50 = 100 channels
25 KHz
5,000 KHz
Reverse Channel Forward Channel
1 2 3 100 1 2 3 100
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Now assume:
Number of channels C =100 Traffic intensity generated by each user:
μ: call request rate: 2 calls/hour H: Holding time: 3 minutes/call Au = μ x H = 2 x (3/60) = 0.1 Erlangs
Grade of Service: 1% Blocked calls cleared (Erlang-B formula) How many users can this system support?
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How many users?
A = 84.1 Erlangs U = A / Au = 84.1 Erlangs / 0.1 Erlangs/user =
= 841 users
Number of channels C Capacity (Erlangs) for GOS=0.01
100 84.1
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Example – Cellular System If a total of 5 MHz of bandwidth is allocated to a
particular FDD cellular wireless system which uses 25 KHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses 4-cell reuse 7-cell reuse
25 KHz
5,000 KHz
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Solution Total bandwidth = 5 MHz Channel bandwidth = 25 KHz x 2 simplex channels = 50
KHz/duplex channel Total available channels = 5,000/50 = 100 channels
A) for N=4 Total number of channels per cell = 100/4 = 25 channels
B) for N=7 Total number of channels per cell = 100/7 ~14 channels
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For N=7:
Number of channels per cell C =14 Traffic intensity generated by each user:
μ: call request rate: 2 calls/hour H: Holding time: 3 minutes/call Au = μ x H = 2 x (3/60) = 0.1 Erlangs
Grade of Service: 1% Blocked calls cleared (Erlang-B formula) How many users per cell?
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For N=7: (cont.)
From the Erlang B chart:Total carried traffic A = 9 ErlangsNumber of users/cell
U = A/ Au = 9/0.1 = 90 users If there are 100 cells in a metropolitan area,
then the total number of subscribers is 9000.Exercise:
Do the calculations for N=4